1. Field of the Invention
The present invention relates to an image forming apparatus such as a color copying machine or a color laser printer.
2. Description of the Related Art
As shown in FIG. 24, an image forming apparatus 1001 which is a conventional image forming apparatus is comprised of a transfer belt 1005 as a transfer material bearing member that carries and conveys transfer materials P. In the image forming apparatus 1001, process cartridges (hereinafter merely referred to as the cartridges) for yellow (Y), magenta (M), cyan (C), and black (Bk) are arranged in tandem along a transfer material bearing surface of the transfer belt 1005. Above the cartridges 1014 to 1017, optical units 1018, 1019, 1020, and 1021 are arranged in association with the respective cartridges 1014 to 1017. Below the cartridges 1014 to 1017, transfer rollers 1010, 1011, 1012, and 1013 associated with respective photosensitive drums 1006, 1007, 1008, and 1019 which are image bearing members for the respective cartridges 1014 to 1017 are arranged with the transfer belt 1005 interposed therebetween.
With the above arrangement, yellow, magenta, cyan, and black toner images obtained by a known electrophotographic process are transferred in a superposed manner onto a transfer material P fed from a sheet cassette 1002 to the transfer belt 1005 by a pickup roller 1003 and a sheet feed and conveying roller pair 1029. The toner images transferred onto the transfer material P are fixed by a fixing unit 1022 and discharged from the apparatus via a discharged sheet sensor 1024 and a sheet path 1023.
To form toner images on the reverse side of the transfer material P as well, the transfer material P is conveyed to the transfer belt 1005 again via another sheet path 1025 after having passed through the fixing unit 1022, and toner images are formed on the reverse side of the transfer material P in a manner similar to the above described manner.
It should be noted that the transfer belt 1005 is rotatively driven by a transfer belt drive roller 1004.
In the image forming apparatus 1001, the optical units 1018 to 1021 for the respective colors scan the surfaces of the respective photosensitive drums 1006 to 1009 by exposing them to laser beams L1, L2, L3, and L1, whereby latent images are formed on the surfaces of the respective photosensitive drums 1006 to 1009. In the sequence of image forming operations carried out in the image forming apparatus 1001, the laser beams L1, L2, L3, and L4 are controlled to perform scanning in synchronization so that the transfer of images can be started at predetermined locations on a transfer material P being conveyed.
The image forming apparatus 1001 is comprised of a sheet feed and conveying motor that drives the sheet feed and conveying roller pair 1029, a transfer belt drive motor that drives the transfer belt drive roller 1004, a photosensitive drum drive motor that drives the respective color photosensitive drums 1006 to 1009, a fixing roller drive motor that drives a fixing roller pair 1022a of the fixing unit 1022, and so on (none of them is illustrated). In order to form satisfactory images, these motors are controlled to fixed rotational speeds.
With the conventional image forming apparatus, however, there may be a case where the inside temperature is increased due to temperature control of a heater incorporated in the fixing unit and/or heating of the drive motors, and the transfer belt drive roller thermally expands due to the increased temperature, causing an increase in the speed of the transfer belt. In such a case, when toner images of respective colors are transferred in a superposed manner onto a specific position on a transfer material, so-called color shift occurs to cause significant degradation of image quality. Specifically, since the photosensitive drums and the transfer belt drive roller are controlled to rotate at fixed speeds, the circumferential velocity of the transfer belt drive roller increases as the diameter of the transfer belt drive roller increases due to thermal expansion, and as a result, the speed of the transfer belt increases, which causes color shift.
As an example of methods to solve such a problem, there is a method in which a color shift detecting pattern is formed on the transfer belt and read by a sensor to detect the relative amounts of color shift of respective colors, and the start positions of image writing by laser beams for the respective colors are corrected based on the detection result, that is, registration correction is carried out. This method, however, has the following problems:
(1) Although the start positions of image writing for respective colors can be in registration immediately after registration correction, the circumferential velocity of the transfer belt gradually increases, for example, in the case of continuous printing since the temperature in the apparatus further increases, and thus, after completion of printing on a plurality of sheets, the amount of color shift is large.
(2) To solve this problem, for example, registration correction may be carried out each time printing on a predetermined number of sheets is completed, but frequent registration correction would decrease the throughput of the image forming apparatus. In addition, since a registration correcting pattern is formed on the transfer belt during registration correction, the consumption of toners is increased, which causes degradation of cost-efficiency for users.
As another example of methods to correct for color shift, there is a method in which a registration reference mark is formed in advance on the transfer belt and detected by a CCD sensor, and image writing positions are corrected based on the detection result (see Japanese Laid-Open Patent Publication (Kokai) No. 2000-071522, for example).
In this method, however, due to the need for forming the reference mark in advance on the transfer belt, the manufacturing cost of the transfer belt is high, and the apparatus has to be wide so as to ensure a space for the reference mark on the transfer belt.
Such a problem also arises in an image forming apparatus having an intermediate transfer member.
Further, the conventional image forming apparatus is provided with the sheet feed and conveying roller pair for feeding and conveying transfer materials. When the speed of the transfer belt increases with an increase in temperature in the apparatus, a difference between the transfer material conveying force exerted by the sheet feed and conveying roller pair and the transfer material conveying force exerted by the transfer belt increases, causing color shift and image blurring. When the transfer material conveying force exerted by the sheet feed and conveying roller pair is greater than the transfer material conveying force exerted by the transfer belt, a transfer material is more likely to be pushed in the conveying direction. In this case, image blurring occurs at the trailing end of the transfer material if the transfer material that is relatively elastic, such as thick paper.
On the other hand, there is the problem that, when the transfer material conveying force exerted by the transfer belt is greater than the transfer material conveying force exerted by the sheet teed and conveying roller pair, image blurring or color shift occurs at the leading end of the transfer material.